So that we can understand which of our websites are generating what amount of revenue or even the amount of revenue particular pages or sections within one website are generating, inside your account you can set up channels which will enable you to see where your Adsense clicks are coming from.

There are two principal types of channel:

1. URL Channels
Unlike custom channels, with url channels you don't need to include a channel id in the actual Adsense code. Instead, simply set up a new url channel inside your account for a particular domain, subdomain, directory or page and Google's technology will automatically report the Adsense performance of the pages inside that channel.

The examples Google gives are:

example.com track all pages across all subdomains
sports.example.com track only pages across the 'sports' subdomain
sports.example.com/widgets track all pages below a specific directory
sports.example.com/index.html track a specific page

2. Custom Channels to track the performance of a particular ad that you have generated
With these channels the id for the particular channel you create is included in the actual Adsense ad code that you generate. Every ad that you include this code in will report its performance under that particular channel even if they are different format sizes or appear on different pages or different websites.

The amount of channels you are allowed to create is currently 200. Related reading includes the best practices for channels and how to analyze your Adsense reports.